1. Field of the Invention
This invention relates generally to piezoelectric acoustic wave devices and more particularly to adjustment of the resonant frequency of piezoelectric acoustic wave devices.
2. Description of the Prior Art
The existence of an effect referred to as the polarizing or electroacoustic effect is well known to those skilled in the art and occurs in all piezoelectric crystals. Whereas piezoelectricity produces strains when an electric field is applied, the electroacoustic effect changes the effective elastic constants by a small amount which operate to vary the acoustic velocity of surface acoustic waves, shallow bulk acoustic waves and reflected bulk acoustic waves being propagated in the body of the crystal. The overall result is a change in the resonant frequency of such devices or delay times provided thereby. So far as known, however, the effects are relatively small but is known to be larger in strong piezoelectric materials such as lithium niobate and lithium tantalate than in quartz which is a relatively weak piezoelectric material. The strength or weakness of the effect, insofar as presently known, appears to be due to the uniform electric field used. In bulk wave resonators, this appears natural since the electrodes produce the uniform field of a parallel capacitor. In surface acoustic wave devices, the effect which is relatively weak is due to the use of auxiliary electrodes that are conventionally used to produce the uniform field.
It is an object of the present invention, therefore, to provide an improvement in piezoelectric acoustic wave devices.
It is a further object of the invention to provide a method and apparatus for adjusting the characteristic parameters of piezoelectric acoustic wave devices.
It is another object of the invention to provide a method and means for compensating for the effects of environmentally caused changes in aging on piezoelectric acoustic wave devices.
It is still a further object of the invention to provide a method and apparatus for providing a deliberate adjustment in variation of certain parameters for adjusting the resonant frequency of piezoelectric acoustic wave devices.